In a fixed telephone network or mobile network, emergency calls are given a higher priority than normal calls to ensure the greatest possibility that they are completed successfully. In the event that an emergency call completes to a public safety answering point (“PSAP”) center and then subsequently is disconnected prematurely, it is the obligation of the PSAP center to call back to the originator of the emergency call.
A potential conflict exists between this requirement and advanced telephony services provided by fixed wire-line and mobile wireless operators that perform any form of mediation on incoming calls (e.g. caller screening, selective call blocking, etc.). For example, a prepaid service that bars call completion to subscribers with a zero balance in their account may inadvertently block a PSAP center call to a subscriber who initiated an emergency call that was dropped.
The issue that arises is that a mobile or fixed line carrier can easily recognize an initiated emergency call by the dialed digits, “911” in North America, “112” in Europe, yet no easy or reliable method is available to identify the PSAP center numbers, therefore services that mediate incoming calls may inadvertently block PSAP center call backs. To avoid conflicts with PSAP call back, incoming calls are not screened. However, failure to screen incoming call limits the effectiveness of any potential call screening service.
In another approach, a processing server at an advanced screening service maintains a number list of all PSAP centers. All calls subject to possible blocking are screened according to the list. If the incoming call number is recognized as a PSAP center, the call is completed immediately without applying any further call screening services. However, lists of appropriate PSAP numbers are difficult to maintain. PSAP centers with multiple telephone lines change numbers on a regular basis. In smaller rural regions, the emergency calls are very often routed to a local sheriff using a cell phone; this assignment can change on a daily basis based on shift rotations in the sheriff's office. These remote office lines, agency issued mobile devices or personal devices may not be recorded in the PSAP call center database.